A Windows tracker for the SN76489 sound chip, which was used in the Sega Master System and Sega Game Gear... among other things. The chip has three square waves and one noise channel.
MOD2PSG2 can import: .mod, .mid, .vgm, and its own save format of .psgmod files. It can export .mid and .vgm.
The program is relatively user friendly, but has a bit of a learning curve for inputs. Be sure to take a look at the readme!